This invention relates to MW(Millimeter Wave)/FIR(Far Infra Red) light detectors for detecting video signals in the MW and FIR wavelength range using a MW/FIR measuring instrument, especially by controlling semiconductor quantum dots.
In general, detectors for electromagnetic waves include a frequency mixer that applies phase sensing wave detection and a video signal detector that adopts incoherent wave detection, of which the latter is known to provide higher sensitivity in detecting a feeble or weak light.
Of the conventional video signal detectors for such lights in an MW/FIR wavelength range, those that offer best sensitivities are a germanium composite bolometer for use at a cryogenic temperature of 0.3 K or lower for a light of a wavelength in the range of 0.1 to 1 mm, and a germanium doped photoconductive detector for use at a low temperature around 2 K for a light of a wavelength in the range of 0.06 to 0.1 mm.
These detectors provide noise equivalent powers (NEP) that reach as high as 10xe2x88x9216 to 10xe2x88x9218 WHZxe2x88x92xc2xd.
This as seen in terms of energy quanta of electromagnetic waves or photons means that the sensitivity of such a detector is such that in one second of measurement, the detector cannot detect a signal as more than a noise unless photon packets of about one million or more in number are incident on the detector.
In addition, such a detector has a speed of response as very low as 100 millisecond. While slow response detectors such as a superconducting bolometer, superconducting tunnel junction and hot electrons in a semiconductor (InSb) have been utilized, their sensitivities fall below that of a germanium composite bolometer.
Apart from the detectors mentioned above, it has been known that irradiating a single-electron transistor with a microwave gives rise to a signal by photon assisted tunneling effect. However, a detector that utilizes this effect is low in sensitivity because between the electrodes no more than one electron moves by absorption of one electromagnetic-wave photon.
Thus, there has so far been no detector that is excellent in both sensitivity and speed of response. This is for the reasons that in any of the detectors, conduction electrons because of lying in a continuous energy band structure are short in the life in which they remain excited by an electromagnetic wave; that since a detector detects an electromagnetic wave in terms of a change in electrical conductance by all the electrons in the detector, an effect brought about by the excitation of a small number of electrons is weakened by the other electrons overwhelming in number; and further that as in the photon assisted tunneling, between the electrodes no more than one electron moves by absorbing one electromagnetic-wave photon.
It is accordingly an object of the present invention to circumvent resolving the problems encountered by the conventional detectors and to provide MW/FIR light detectors predicated on principles or mechanisms totally different from those mentioned above, which detectors have an extraordinary degree of sensitivity and are quick in response.
In order to achieve the object, mentioned above, there is provided in accordance with the present invention in one form of embodiment thereof an MW(millimeter wave)/FIR(infra red) light detector that comprises an electromagnetic-wave coupling means for concentrating an electromagnetic wave in a small special region of a submicron size a quantum dot for absorbing the concentrated electromagnetic wave to bring about an excited state between electron levels, and a single-electron semiconductor.
In addition to the make-up mentioned above an MW/FIR detector according to the present invention preferably retains a state in which an electrical conductance of the said single-electron semiconductor is varied according to the said excited state of the quantum dot.
In an MW/FIR detector as mentioned above, the said quantum dot preferably has a life in a range of 10 nanoseconds to 1000 seconds in which it remains in the said excited state before returning to a ground state thereof.
According to one specific feature of the present invention, the said electron levels have a difference in energy that is controllable variably according to any one or a combination of a change in size of the said quantum dot, an external magnetic field and a biasing voltage.
According to another specific feature of the present invention, the said excited state is established by any one or a combination of a resonance excitation of electrons according to a size effect of the said quantum dot, a resonance excitation of electrons between Landau levels by application of a magnetic field and an excitation between spin states separated by a magnetic field.
For the said electromagnetic-wave coupling means use may be made of a standard or regular BOTAI antenna for electrically coupling the said quantum dot and the said electromagnetic wave together.
For the said electromagnetic-wave coupling means, use may also be made of an anomalous or irregular BOTAI antenna having an node thereof short-circuited for magnetically coupling the said quantum dot and the said electromagnetic wave together.
Preferably, the presence or absence of short circuit through a node of the said electromagnetic-wave coupling means and the size of the said quantum dot are determined according to the wavelength of the said electromagnetic wave.
The said electromagnetic-wave coupling means may be used also to provide a gate electrode for the said single-electron transistor.
The present invention provides in a second form of embodiment, thereof an MW/FIR light detector, characterized in that the detector comprises: an electromagnetic-wave coupling means for concentrating an electromagnetic wave in a small special region of a sub-micron size; a first quantum dot for absorbing the electromagnetic wave concentrated by the said electromagnetic-wave coupling means to bring about an ionization thereof; and a single-electron transistor including a second quantum dot electrostatically coupled to the said first quantum dot, whereby the said electromagnetic wave is detected on the basis of the fact that electric conductivity of the said single-electron transistor varies with a change in electrostatic state of the said second quantum dot consequent upon an ionization of the said first quantum dot.
The above mentioned ionization of the said first quantum dot may be brought about by excitation of an electron in a quantized bound state of the said first quantum dot to a free electron state of an electron system outside of the said first quantum dot.
The ionization energy of the said first quantum dot may be controllable variably by changing the magnitude of a bias voltage applied to a gate of the said first quantum dot.
The said first quantum dot may have a life in a range between 1 microsecond and 1000 seconds in which it remains in the ionization state before returning to a neutral state.
The said first and second quantum dots preferably lie in an identical semiconductor structure and are isolated from each other electrostatically by bias voltages applied to respective gates thereof, respectively.
The said first and second quantum dots may be formed adjacent to each other across a gap in a semiconductor.
Preferably, the said second quantum dot comprises a metal dot formed on the said first quantum dot and forms the said single-electron transistor by having a tunnel junction with a metal lead wire formed on the said metal dot.
Then, the said second quantum dot preferably an aluminum metal dot and has a portion of a said tunnel junction formed from aluminum oxide.
The said electromagnetic-wave coupling means may be a standard dipole antenna for electrically coupling the said first quantum dot and the said electromagnetic wave together.
The said electromagnetic-wave coupling means may be used also to serve as a bias voltage applying gate that forms the said first and second quantum dots.
The said electromagnetic-wave coupling means preferably has a lead portion oriented longitudinally in a direction that is perpendicular to a direction of the axis of polarization of the said electromagnetic-wave coupling means.
The node of the said electromagnetic-wave coupling means preferably is substantially equal in size to a maximum size of a said quantum dot.
The said electromagnetic-wave coupling means may have an electrode diameter that is about one half less in length than the wavelength of the said electromagnetic wave.
The said single-electron transistor may have a single hetero structure that forms a two-dimensional electron system and a said quantum dot may be formed by electrically confining a two-dimensional electron gas by a gate electrode of the said single-electron transistor.
The said single-electron transistor preferably comprises a single hetero structure that forms a two-dimensional electron system a gate electrode for controlling electrostatic potential of a said quantum dot tunnel coupled via to the said two-dimensional electron system, and a source and a drain electrode that form a source and a drain region, respectively, which are tunnel coupled to the said quantum dot.
The said single-electron transistor preferably includes a gate electrode for controlling source-drain electric current and a gate electrode for forming a said quantum dot.
The source electrode and the drain electrode of the said single-electron transistor preferably are apart from each other by a distance that is not less than the length of the said electromagnetic-wave coupling means in a direction of its axis of polarization.
The said single-electron transistor comprises a compound semiconductor, especially a III-V group compound semiconductor.
For the said single-electron transistor, preference is also given of a III-V group compound semiconductor superlattice selection doped, single hetero structure.
The said single-electron transistor preferably has a aluminum-gallium arsenide/gallium arsenide selection doped, single hetero structure.
The said single-electron transistor preferably comprises a IV group semiconductor.
The said single-electron transistor preferably is formed symmetrically about a said quantum dot.
An MW/FIR light detector according to the second form embodiment of the present invention preferably further includes a light introducing means for guiding the said electromagnetic wave into the said electromagnetic-wave coupling means.
According to an MW/FIR light detector of the present invention constructed as mentioned above, an electromagnetic wave to be detected is efficiently concentrated in a quantum dot by an electromagnetic-wave coupling means, and a resonance excitation brought about between electron levels in the quantum dot by absorbing the electromagnetic wave is detected upon amplification by a single-electron transistor.
If the detecting means is a standard or regular BOTAI (i.e., xe2x80x9cbow tiexe2x80x9d) antenna, an excitation is brought about electrically by transition in the quantum dot. If it is an anomalous or irregular BOTAI antenna, an excitation is magnetically brought about in the quantum dot.
Also, if the quantum dot of the single-electron transistor is with an aluminum-gallium arsenide/gallium arsenide selection doped, single hetero structure crystal, it is a small dot having an effective diameter in a two-dimensional electron system ranging from 0.02 xcexcm to 0.6 xcexcm.
Serving the electromagnetic-wave coupling means as a gate electrode of the single-electrode transistor couples the quantum dot weakly to a two-dimensional electron system in its outside via a tunnel junction.
In this way, the present invention enables the energy of an electromagnetic wave to be converged and absorbed in a quantum dot of a size that, is one hundredth or less smaller than the wavelength of the electromagnetic wave and then the excited state brought about to be retained for 1.0 nanoseconds or more.
As a consequence, a change in electrical conductivity caused by absorption of one electromagnetic photon is kept for 10 nanoseconds or more. Although the time constant of a single-electron transistor when operated is in actuality circumscribed by an amplifier used, constructing a current amplifier circuit by combining a HEMT amplifier cooled to a helium (liquefier, refrigerator or cooling) temperature and an LC tank circuit permits such a change in conductivity to be measured in a time constant of three (3) nanoseconds. Therefore, detecting a single photon can be actualized under a practical condition.
Also, in case a pair of separate quantum dots, i.e., a first quantum dot for absorbing an electromagnetic wave and a second quantum dot which is conductive, for detection are used, a positive bole and an electron that are excited upon absorbing an electromagnetic energy are created separately in the inside and outside of the first quantum dot. This enables an extremely prolonged state of excitation, hence life of ionization to be established without the need to apply a magnetic field. Therefore, a rise in sensitivity is achieved without the need to use a magnetic field while permitting a single photon to be readily detected.
Further, in an electron system that constitutes the first quantum dot there exists a threshold value for utilizing excitation from a discrete level to a continuous band level, to wit, a continuous wavelength range that possesses an amount of energy in excess of the ionization energy and thus offers good detection sensitivity. The threshold wavelength, to wit, the ionization energy can also be controlled directly through the adjustment of the height of the potential barrier by the gate voltage.
It has further been found that reducing the second quantum dot in size permits the operating temperature to be raised up to a maximum of 2 K.
An MW/FIR light detector according to the present invention makes uses of a single-electron transistor (hereinafter referred to also as xe2x80x9cSETxe2x80x9d) by a semiconductor quantum dot. A SET possesses a single hetero structure of a semiconductor superlattice that forms a two-dimensional electron gas, for example. It is formed of a dot that is a very small isolated conductive region weakly coupled through a tunnel junction to a source and a drain region by a source and a drain electrode, and is provided with a control gate electrode for controlling the electrostatic potential of the dot.
It should be noted further that the SET may comprise a compound semiconductor, especially a compound semiconductor of a III-V group compound, and may have a selection doped, single hetero structure with a III-V group compound semiconductor superlattice. Further, in the case of a plurality of quantum dots used in forming an MW/FIR light detector of the present invention, the SET may be a semiconductor of a compound of the IV group.
If the bias voltage of the control gate electrode is varied, the electrochemical potential of a conduction electron in the dot will vary. Then, a source-drain current ISD will flow only under the condition that the same is equal to the Fermi energy of the source and drain electrodes.
The conductivity of a SET in its such conductive state G=ISD/VSD in general becomes [100-400 Kxcexa9]xe2x88x921. Here, VSD represents a source-drain voltage of the SET, which must be set at not more than 100 xcexcV in the present invention.
If for the conductive dot, use is made of a semiconductor quantum dot whose effective size is 0.02 to 0.6 xcexcm in diameter, the energy level of its internal electron system will be quantized by its size effect and according to a magnetic field applied externally. And its energy level spacing then corresponds to a light quantum in a MW/FIR light region. That energy level spacing can be controlled by changing the size of the quantum dot, or externally applying a magnetic field or a bias voltage. Accordingly, it, becomes possible to excite electrons resonantly inside the quantum dot by irradiating it with an MW/FIR light. However, as described later, the state excited varies depending on the way of excitation and the presence or absence of a magnetic field applied.
In either the case, since the wave function of the excited electrons in their special symmetry and distribution varies from the wave function of electrons in their ground state, the electrochemical potential of the quantum dot and the intensity of its tunnel coupling to source and drain regions vary to a large extent. For this reason, the excitation of one electron alone in the semiconductor quantum dot causes the conductivity of the SET to, vary as largely as 20 to 99% and permits the state that the conductivity is varied to be retained until the excited state diminishes and returns to the ground state, to wit, for the life of the states of excitation and its relaxation.
On the other hand, the excited quantum dot because of its structure of discrete energy levels has its life as long as 10 nanoseconds to 1000 seconds before returning to its ground state and hence becomes a detector that is extremely high in sensitivity. The changes in number of the electrons fed from the source electrode into the drain electrode, N=GVSDT(X/100)/e, where a change X% in the conductivity lasts for T seconds, are as numerous as 106 under a typical condition that G=1/300 kxcexa9, X=50%, T=1 mseconds and VSD=0.05 mV. Thus, absorbing one photon can transport electrons as many as one millions in number or more.
Moreover, the time constant CSD/G of operation of a SET in principle is as extremely short as several tens pico-seconds, where CSD is an electrostatic capacitance between source and drain electrodes. It thus becomes possible to detect a single MW/FIR photon by way of quick time splitting measurement of an electric current.